An organic electroluminescence (EL) device is a self-emission device utilizing the principle that light is emitted by the recombination energy of holes injected from an anode and electrons injected from a cathode. In order to outcouple the light generated by recombination to the outside of the device, in general, a conductive film having light transmittance is used in one or both of an anode and a cathode.
As such a conductive film, an indium-tin-oxide film (Indium-Tin-Oxide: ITO) is preferably used. Since ITO has a work function of about 5 eV, it is preferable to be used as an anode. On the other hand, ITO is not suited to be used as a cathode, since difference in affinity level with surrounding layers such as an electron-transporting layer is large.
Therefore, in an organic EL device in which light is outcoupled from a cathode, it has been proposed that formation of an electron-injecting layer formed of a mixture of a metal having a small work function such as cesium (Cs) and an electron-transporting organic material between an organic emitting layer and an electrode formed of a transparent conductive film. Due to such a configuration, electron-injection properties of a cathode are enhanced.
Many of transparent conductive films represented by ITO are made of a metal oxide, and formed by a sputtering method using argon (Ar) or oxygen (O2) as a process gas. At this time, the electron-injecting layer containing the above-mentioned metal having a small work function is decomposed and oxidized, resulting in lowering in electron-injection properties. As a result, problems arise that the driving voltage of a device is increased, current leakage occurs, device life is shortened, or the like.
In order to solve the problem, an organic EL device using copper phthalocyanine as the underlayer of the transparent conductive film has been disclosed (see Patent Document 1).
An organic EL device in which a stacked body of a layer obtained by contacting an organic metal complex compound containing at least one of ions of small-work-function metals having a work function of 4.0 eV or less and a heat-reductive metal such as aluminum (Al) and a layer formed of molybdenum oxide, porphyrin or the like is formed in an electron-transporting part and a transparent conductive film is formed thereon (see Patent Document 2, FIG. 19).
Further, an organic EL device is disclosed in which a donor-containing layer, an acceptor-containing layer and a cathode are provided in this sequence and the donor-containing layer contains at least one selected from a donor metal, a donor metal compound and a donor metal complex (see Patent Document 3).
However, when comparing with a device using a cathode of a metal having a small work function formed by deposition, this device has a problem that the efficiency of a device is deteriorated and the life of a device is shortened.